The majority of the hearing impaired individuals who visit hearing clinics and hearing instrument dispensers are persons suffering from high frequency hearing loss caused by presbycusis, noise-induced hearing loss or combinations of the two. The consequence of such hearing loss is reduced understanding of speech, particularly in noisy environments. This is because the hearing impairment is located in the frequencies most important for the understanding of speech, as well as in the speech frequencies with the lowest sound intensity.
High frequency hearing impairments develop gradually and often go unnoticed for many years. In addition, the patients have almost normal hearing at low frequencies, which gives them the impression of loudness without contributing to understanding. Consequently, these individuals often are not very motivated to use hearing instruments. Frequently, they claim that they hear well enough if others would only speak more distinctly. If their impairment is not remedied, these people risk having to live an isolated life with very limited social contact.
Persons suffering from presbycusis or noise-induced hearing loss are reluctant to accept hearing instruments or flatly refuse to wear them at all. They claim that the sound is too sharp and too noisy. This is because their hearing impairment has developed over a long period of time, causing them to gradually become accustomed to a sense of hearing in which high frequencies are missing. When fitting a hearing instrument with the correct characteristics for this kind of hearing loss, the sudden renewed confrontation with high frequencies can be a very unpleasant experience, even if improved speech discrimination in quiet environments is observed. Usually, the hearing specialist has to make a very difficult compromise between acceptance and optimum speech discrimination. In fact, such cases call for a gradual compensation of the high frequency part of the hearing loss. Using ordinary hearing instruments, this can be a protracted and very complicated process, since the wearer's individual psyche and patience, as well as the actual acoustic environment, have to be taken into account.
On the basis of measurements made on a large number of hearing impaired with high-frequency hearing loss, one can plot curves of the hearing loss as a function of the frequency, with the peoples' ages as a parameter, which is shown stylized in FIG. 1 of the drawing. These and other corresponding curves can be seen in many textbooks and periodicals concerning the subject. At frequencies below approximately 1000 Hz, the hearing loss is not dependent on frequency, while at the higher frequencies it is frequency-dependent, as can be seen quite clearly in FIG. 1. From this it can be seen that a large number of people in the age group of 30-60 years have a frequency-dependent hearing loss above approximately 1000 Hz, which can give hearing problems, while those below 1000 Hz have almost normal hearing. Above approximately 1000 Hz, the higher the age of the person, the greater the slope.